High hydrocarbon resistant chemically cross-linked aromatic polyimide membrane for separations

ABSTRACT

This invention relates to high hydrocarbon resistant chemically cross-linked aromatic polyimide polymers, membranes and methods for making and using these polymers and membranes. The high hydrocarbon resistant chemically cross-linked aromatic polyimide membrane described in the present invention comprises a plurality of repeating units of a first aromatic polyimide comprising hydroxyl groups cross-linked with a second aromatic polyimide comprising carboxylic acid groups via covalent ester bonds. These membranes exhibit high permeability and selectivity in separation of mixtures of gases and liquids.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of application Ser. No. 13/929,864 filedJun. 28, 2013, which issued as U.S. Pat. No. 9,126,154 on Sep. 8, 2015,the contents of which are hereby incorporated by reference in itsentirety

BACKGROUND OF THE INVENTION

This invention relates to high hydrocarbon resistant chemicallycross-linked aromatic polyimide membranes and methods for making andusing these membranes.

In the past 30-35 years, the state of the art of polymer membrane-basedgas separation processes has evolved rapidly. Membrane-basedtechnologies have advantages of both low capital cost and high-energyefficiency compared to conventional separation methods. Membrane gasseparation is of special interest to petroleum producers and refiners,chemical companies, and industrial gas suppliers. Several applicationsof membrane gas separation have achieved commercial success, includingN₂ enrichment from air, carbon dioxide removal from natural gas and fromenhanced oil recovery, and also in hydrogen removal from nitrogen,methane, and argon in ammonia purge gas streams. For example, UOP'sSeparex™ cellulose acetate spiral wound polymeric membrane is currentlyan international market leader for carbon dioxide removal from naturalgas.

Polymers provide a range of properties including low cost, permeability,mechanical stability, and ease of processability that are important forgas separation. Glassy polymers (i.e., polymers at temperatures belowtheir T_(g)) have stiffer polymer backbones and therefore let smallermolecules such as hydrogen and helium pass through more quickly, whilelarger molecules such as hydrocarbons pass through more slowly ascompared to polymers with less stiff backbones. Cellulose acetate (CA)glassy polymer membranes are used extensively in gas separation.Currently, such CA membranes are used for natural gas upgrading,including the removal of carbon dioxide. Although CA membranes have manyadvantages, they are limited in a number of properties includingselectivity, permeability, and in chemical, thermal, and mechanicalstability.

The membranes most commonly used in commercial gas and liquid separationapplications are asymmetric polymeric membranes and have a thinnonporous selective skin layer that performs the separation. Separationis based on a solution-diffusion mechanism. This mechanism involvesmolecular-scale interactions of the permeating gas with the membranepolymer. The mechanism assumes that in a membrane having two opposingsurfaces, each component is sorbed by the membrane at one surface,transported by a gas concentration gradient, and desorbed at theopposing surface. According to this solution-diffusion model, themembrane performance in separating a given pair of gases (e.g., CO₂/CH₄,O₂/N₂, H₂/CH₄) is determined by two parameters: the permeabilitycoefficient (abbreviated hereinafter as permeability or P_(A)) and theselectivity (α_(A/B)). The P_(A) is the product of the gas flux and theselective skin layer thickness of the membrane, divided by the pressuredifference across the membrane. The α_(A/B) is the ratio of thepermeability coefficients of the two gases (α_(A/B)=P_(A)/P_(B)) whereP_(A) is the permeability of the more permeable gas and P_(B) is thepermeability of the less permeable gas. Gases can have high permeabilitycoefficients because of a high solubility coefficient, a high diffusioncoefficient, or because both coefficients are high. In general, thediffusion coefficient decreases while the solubility coefficientincreases with an increase in the molecular size of the gas. In highperformance polymer membranes, both high permeability and selectivityare desirable because higher permeability decreases the size of themembrane area required to treat a given volume of gas, therebydecreasing capital cost of membrane units, and because higherselectivity results in a higher purity product gas.

One of the components to be separated by a membrane must have asufficiently high permeance at the preferred conditions orextraordinarily large membrane surface areas is required to allowseparation of large amounts of material. Permeance, measured in GasPermeation Units (GPU, 1 GPU=10⁻⁶ cm³ (STP)/cm²s (cm Hg)), is thepressure normalized flux and equals to permeability divided by the skinlayer thickness of the membrane. Commercially available gas separationpolymer membranes, such as CA, polyimide, and polysulfone membranesformed by phase inversion and solvent exchange methods have anasymmetric integrally skinned membrane structure. Such membranes arecharacterized by a thin, dense, selectively semipermeable surface “skin”and a less dense void-containing (or porous), non-selective supportregion, with pore sizes ranging from large in the support region to verysmall proximate to the “skin”. However, fabrication of defect-free highselectivity asymmetric integrally skinned polyimide membranes isdifficult. The presence of nanopores or defects in the skin layerreduces the membrane selectivity. The high shrinkage of the polyimidemembrane on cloth substrate during membrane casting and drying processresults in unsuccessful fabrication of asymmetric integrally skinnedpolyimide membranes using phase inversion technique.

In order to combine high selectivity and high permeability together withhigh thermal stability, new high-performance polymers such as polyimides(PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole weredeveloped. These new polymeric membrane materials have shown promisingproperties for separation of gas pairs like CO₂/CH₄, O₂/N₂, H₂/CH₄, andC₃H₆/C₃H₈. However, current polymeric membrane materials have reached alimit in their productivity-selectivity trade-off relationship. Inaddition, gas separation processes based on glassy polymer membranesfrequently suffer from plasticization of the stiff polymer matrix by thesorbed penetrating molecules such as CO₂ or C₃H₆. Plasticization of thepolymer is exhibited by swelling of the membrane structure and by asignificant increase in the permeances of all components in the feed anddecrease of selectivity occurring above the plasticization pressure whenthe feed gas mixture contains condensable gases. Plasticization isparticularly an issue for gas fields containing high CO₂ concentrationsand heavy hydrocarbons and for systems requiring two-stage membraneseparation.

US 2005/0268783 A1, US 2009/0182097 A1, and US 2009/0178561 A1 disclosedchemically cross-linked polyimide hollow fiber membranes prepared fromtwo separate steps. Step one is the synthesis of a monoesterifiedpolyimide polymer in a solution by treating a polyimide polymercontaining carboxylic acid functional group with a small diol moleculeat esterification conditions in the presence of dehydrating conditions.However, significant extra amount of diol was used to prevent theformation of biesterified polyimide polymer. Step two is the solid statetransesterification of the monoesterified polyimide membrane at elevatedtemperature to form a cross-linked polyimide membrane.

Chemical cross-linking of polyimides using diamine small molecules wasalso disclosed. (J. MEMBR. SCI., 2001, 189, 231-239) However, CO₂permeability decreased significantly after this type of cross-linking.In addition, the thermal stability and hydrolytic stability of thediamine cross-linked polyimide were not improved.

Koros et al. disclosed decarboxylation-induced thermally cross-linkedpolyimide membrane. (J. MEMBR. SCI., 2011, 382, 212-221) However,decarboxylation reaction among the carboxylic acid groups on thecarboxylic acid group-containing polyimide membrane occurred attemperatures higher than the glass transition temperature of thepolyimide polymer. Such a high temperature resulted in densification ofthe substructure of the membrane and decreased membrane permeance.

U.S. Pat. No. 7,485,173 disclosed UV cross-linked mixed matrix membranesvia UV radiation. The cross-linked mixed matrix membranes comprisemicroporous materials dispersed in the continuous UV cross-linkedpolymer matrix.

U.S. Pat. No. 4,931,182 and U.S. Pat. No. 7,485,173 disclosed physicallycross-linked polyimide membranes via UV radiation. The cross-linkedmembranes showed improved selectivities for gas separations. However, itis hard to control the cross-linking degree of the thin selective layerof the asymmetric gas separation membranes using UV radiation technique,which will result in very low permeances although the selectivities arenormally very high.

The present invention discloses a new type of high hydrocarbon resistantchemically cross-linked aromatic polyimide membranes and methods formaking and using these membranes.

SUMMARY OF THE INVENTION

The present invention is for a chemically cross-linked aromaticpolyimide polymer and a membrane made from this polymer. The polymercomprises a formula (I)

wherein R is selected from the group consisting of —H, —COCH₃,

and mixtures thereof; X1, X2, X3, and X4 are selected from the groupconsisting of

and mixtures thereof, respectively; X1, X2, X3, and X4 are the same ordifferent from each other; Y2 is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; Y1 is selected from the group consisting of

and mixtures thereof; Y3 is selected from the group consisting of

and mixtures thereof, and —R″— is selected from the group consisting of—H, COCH₃, and mixtures thereof; n, m, n′ and m′ are independentintegers from 2 to 500; n/m is in a range of 1:100 to 100:1; and n′/m′is also in a range of 1:100 to 100:1. Preferably, n/m is in a range of1:10 to 5:1; and n′/m′ is also in a range of 1:10 to 5:1.

These polymers are made from a blend of a first aromatic polyimidecomprising carboxylic acid groups comprising a plurality of repeatingunits of formula (II)

wherein X1 and X2 are selected from the group consisting of

and mixtures thereof, and wherein X1 and X2 can be the same or differentfrom each other; wherein Y1 is selected from the group consisting of

and mixtures thereof; wherein n and m are independent integers from 2 to500; and wherein n/m is in a range of 1:100 to 100:1, and preferably n/mis in a range of 1:10 to 5:1, and a second aromatic polyimide comprisinghydroxyl groups comprises a plurality of repeating units of formula(III)

wherein X3 and X4 are selected from the group consisting of

and mixtures thereof, and wherein X3 and X4 can be the same or differentfrom each other;wherein Y2 is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof, and —R″— is selected from the group consisting of—H, COCH₃, and mixtures thereof; wherein Y3 is selected from the groupconsisting of

and mixtures thereof; wherein n′ and m′ are independent integers from 2to 500, and wherein n′/m′ is in a range of 1:100 to 100:1, andpreferably n′/m′ is in a range of 1:10 to 5:1.

The invention further comprises a process for preparing this polymercomprising blending the first aromatic polyimide polymer and the secondaromatic polyimide polymer and then chemical cross-linking the blendpolyimide polymer membrane by heating the membrane at 100 to 300° C.under an inert atmosphere.

The polymer of the invention may be fabricated into any known membraneconfiguration or form. The process of preparing the membrane may furtherinvolve coating a high permeability material onto the membrane such as apolysiloxane, a fluoro-polymer, a thermally curable silicone rubber, ora UV radiation curable epoxy silicone.

The first aromatic polyimide may be selected from the group consistingof poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-3,5-diaminobenzoic acid-2,4,6-trimethyl-m-phenylenediamine)polyimide derived from a polycondensation reaction of3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride with a mixture of3,5-diaminobenzoic acid and 2,4,6-trimethyl-m-phenylenediamine;poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-3,5-diaminobenzoic acid-3,3′,5,5′-tetramethyl-4,4′-methylenedianiline) polyimide derived from a polycondensation reaction of3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride with a mixture of3,5-diaminobenzoic acid and 3,3′,5,5′-tetramethyl-4,4′-methylenedianiline; poly(3,3′,4,4′-benzophenone tetracarboxylicdianhydride-pyromellitic dianhydride-3,5-diaminobenzoicacid-2,4,6-trimethyl-m-phenylenediamine) polyimide derived from apolycondensation reaction of 3,3′,4,4′-benzophenone tetracarboxylicdianhydride and pyromellitic dianhydride with 3,5-diaminobenzoic acidand 2,4,6-trimethyl-m-phenylenediamine; poly(3,3′,4,4′-benzophenonetetracarboxylic dianhydride-pyromellitic dianhydride-3,5-diaminobenzoicacid-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) polyimide derivedfrom a polycondensation reaction of 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and pyromellitic dianhydride with3,5-diaminobenzoic acid and 3,3′,5,5′-tetramethyl-4,4′-methylenedianiline; poly(3,3′,4,4′-benzophenone tetracarboxylicdianhydride-3,5-diaminobenzoic acid-3,3′,5,5′-tetramethyl-4,4′-methylenedianiline) polyimide derived from a polycondensation reaction of3,3′,4,4′-benzophenone tetracarboxylic dianhydride with3,5-diaminobenzoic acid and 3,3′,5,5′-tetramethyl-4,4′-methylenedianiline; poly(pyromellitic dianhydride-3,5-diaminobenzoicacid-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) polyimide derivedfrom a polycondensation reaction of pyromellitic dianhydride with3,5-diaminobenzoic acid and 3,3′,5,5′-tetramethyl-4,4′-methylenedianiline; poly(3,3′,4,4′-benzophenone tetracarboxylicdianhydride-3,5-diaminobenzoic acid-2,4,6-trimethyl-m-phenylenediamine)polyimide derived from a polycondensation reaction of3,3′,4,4′-benzophenone tetracarboxylic dianhydride with3,5-diaminobenzoic acid and 2,4,6-trimethyl-m-phenylenediamine; andpoly(pyromellitic dianhydride-3,5-diaminobenzoicacid-2,4,6-trimethyl-m-phenylenediamine) polyimide derived from apolycondensation reaction of pyromellitic dianhydride with3,5-diaminobenzoic acid and 2,4,6-trimethyl-m-phenylenediamine.

The second aromatic polyimide may be selected from the group consistingof poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl-2,4,6-trimethyl-m-phenylenediamine)polyimide derived from the polycondensation reaction of3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride with a mixture of3,3′-dihydroxy-4,4′-diamino-biphenyl and2,4,6-trimethyl-m-phenylenediamine; poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl]polyimide derived fromthe polycondensation reaction of 2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride with 3,3′-dihydroxy-4,4′-diamino-biphenyl;poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,3′,5,5′-tetramethyl-4,4′-methylenedianiline) polyimide derived from the polycondensation reaction of3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride with a mixture of3,3′-dihydroxy-4,4′-diamino-biphenyl and3,3′,5,5′-tetramethyl-4,4′-methylene dianiline;poly(3,3′,4,4′-benzophenone tetracarboxylicdianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl-2,4,6-trimethyl-m-phenylenediamine)polyimide derived from the polycondensation reaction of3,3′,4,4′-benzophenone tetracarboxylic dianhydride with3,3′-dihydroxy-4,4′-diamino-biphenyl and2,4,6-trimethyl-m-phenylenediamine; poly(3,3′,4,4′-benzophenonetetracarboxylicdianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,3′,5,5′-tetramethyl-4,4′-methylenedianiline) polyimide derived from the polycondensation reaction of3,3′,4,4′-benzophenone tetracarboxylic dianhydride with3,3′-dihydroxy-4,4′-diamino-biphenyl and3,3′,5,5′-tetramethyl-4,4′-methylene dianiline;poly(3,3′,4,4′-benzophenone tetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′,5,5′-tetramethyl-4,4′-methylenedianiline) polyimide derived from the polycondensation reaction of3,3′,4,4′-benzophenone tetracarboxylic dianhydride with2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,3′,5,5′-tetramethyl-4,4′-methylene dianiline.

The invention also involves a process for separating at least one gasfrom a mixture of gases comprising providing the chemically cross-linkedaromatic polyimide blend membrane of formula (I) contacting the mixtureof gases to one side of the chemically cross-linked aromatic polyimidemembrane to cause at least one gas to permeate said membrane; andremoving from an opposite side of said chemically cross-linked aromaticpolyimide membrane a permeate gas composition comprising a portion ofsaid at least one gas that permeated said membrane.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to chemically cross-linkedaromatic polyimide polymers and high hydrocarbon resistant chemicallycross-linked aromatic polyimide membranes for gas, vapor, and liquidseparations, as well as methods for making and using these polymers andmembranes.

The chemically cross-linked polyimide polymers and the high hydrocarbonresistant chemically cross-linked aromatic polyimide membrane describedin the present invention comprise a plurality of repeating units offormula (I), wherein formula (I) comprises a first aromatic polyimidecomprising carboxylic acid groups cross-linked with a second aromaticpolyimide comprising hydroxyl groups via covalent ester bonds. Thechemically cross-linked polyimide polymers and the high hydrocarbonresistant chemically cross-linked aromatic polyimide membrane describedin the present invention comprise aromatic polyimide polymer chainsegments where at least part of these polymer chain segments arecross-linked to each other through direct covalent ester bonds. Theformation of inter-polymer chain cross-linked covalent ester bondsresults in good mechanical stability, excellent resistance tohydrocarbon and high concentration of CO₂. More importantly, thechemically cross-linked aromatic polyimide membrane described in thepresent invention showed high selectivity and high permeability for avariety of gas separation applications such as CO₂/CH₄, H₂/CH₄, andHe/CH₄ separations. For example, a cross-linkedpoly[2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropanedianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl]polyimide (abbreviatedas poly(6FDA-HAB)) blended with poly(3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride-3,5-diaminobenzoicacid-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) polyimidepoly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-3,5-diaminobenzoic acid-3,3′,5,5′-tetramethyl-4,4′-methylenedianiline) polyimide (abbreviated as poly(DSDA-DBA-TMMDA)) membrane hasCO₂ permeance of 8.04 Barrers and high CO₂/CH₄ selectivity of 45.4 forCO₂/CH₄ separation. This chemically cross-linked membrane has H₂permeance of 33.5 Barrers and H₂/CH₄ selectivity of 189 for H₂/CH₄separation. This chemically cross-linked membrane also has He permeanceof 41.8 Barrers and He/CH₄ selectivity of 236 for He/CH₄ separation.

Formula (I) is represented by the following formula:

wherein R is selected from the group consisting of —H, —COCH₃,

and mixtures thereof; X1, X2, X3, and X4 are selected from the groupconsisting of

and mixtures thereof, respectively; X1, X2, X3, and X4 are the same ordifferent from each other; Y2 is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; Y1 is selected from the group consisting of

and mixtures thereof; Y3 is selected from the group consisting of

and mixtures thereof, and —R″— is selected from the group consisting of—H, COCH₃, and mixtures thereof; n, m, n′ and m′ are independentintegers from 2 to 500; n/m is in a range of 1:100 to 100:1; and n′/m′is also in a range of 1:100 to 100:1. Preferably, n/m is in a range of1:10 to 5:1; and n′/m′ is also in a range of 1:10 to 5:1.

The first aromatic polyimide comprising carboxylic acid groups describedin the present invention comprises a plurality of repeating units offormula (II).

wherein X1 and X2 are selected from the group consisting of

and mixtures thereof, and wherein X1 and X2 can be the same or differentfrom each other; wherein Y1 is selected from the group consisting of

and mixtures thereof; wherein n and m are independent integers from 2 to500; and wherein n/m is in a range of 1:100 to 100:1, and preferably n/mis in a range of 1:10 to 5:1

The second aromatic polyimide comprising hydroxyl groups comprises aplurality of repeating units of formula (III).

wherein X3 and X4 are selected from the group consisting of

and mixtures thereof, and wherein X3 and X4 can be the same or differentfrom each other;wherein Y2 is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof, and —R″— is selected from the group consisting of—H, COCH₃, and mixtures thereof; wherein Y3 is selected from the groupconsisting of

and mixtures thereof; wherein n′ and m′ are independent integers from 2to 500, and wherein n′/m′ is in a range of 1:100 to 100:1, andpreferably n′/m′ is in a range of 1:10 to 5:1.

The first and second aromatic polyimide polymers used for making thehigh hydrocarbon resistant chemically cross-linked aromatic polyimidemembrane described in the current invention have a weight averagemolecular weight in the range of 10,000 to 1,000,000 g/mol, preferablybetween 70,000 to 500,000 g/mol.

The weight ratio of the first aromatic polyimide polymer to the secondaromatic polyimide polymer in the high hydrocarbon resistant chemicallycross-linked aromatic polyimide membrane described in the currentinvention is in a range of 10:1 to 1:10.

Some examples of the first aromatic polyimide polymer described in thecurrent invention may include, but are not limited to:poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-3,5-diaminobenzoic acid-2,4,6-trimethyl-m-phenylenediamine)polyimide derived from the polycondensation reaction of3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride (DSDA) with amixture of 3,5-diaminobenzoic acid (DBA) and2,4,6-trimethyl-m-phenylenediamine (TMPDA);poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-3,5-diaminobenzoic acid-3,3′,5,5′-tetramethyl-4,4′-methylenedianiline) polyimide derived from the polycondensation reaction of DSDAwith a mixture of DBA and 3,3′,5,5′-tetramethyl-4,4′-methylene dianiline(TMMDA); poly(3,3′,4,4′-benzophenone tetracarboxylicdianhydride-pyromellitic dianhydride-DBA-TMPDA) polyimide derived fromthe polycondensation reaction of 3,3′,4,4′-benzophenone tetracarboxylicdianhydride (BTDA) and pyromellitic dianhydride (PMDA) with DBA andTMPDA; poly(BTDA-PMDA-DBA-TMMDA) polyimide derived from thepolycondensation reaction of BTDA and PMDA with DBA and TMMDA;poly(BTDA-DBA-TMMDA) polyimide derived from the polycondensationreaction of BTDA with DBA and TMMDA; poly(PMDA-DBA-TMMDA) polyimidederived from the polycondensation reaction of PMDA with DBA and TMMDA;poly(BTDA-DBA-TMPDA) polyimide derived from the polycondensationreaction of BTDA with DBA and TMPDA; poly(PMDA-DBA-TMPDA) polyimidederived from the polycondensation reaction of PMDA with DBA and TMPDA.

Some examples of the second aromatic polyimide polymer described in thecurrent invention may include, but are not limited to:poly(DSDA-3,3′-dihydroxy-4,4′-diamino-biphenyl-TMPDA) polyimide derivedfrom the polycondensation reaction of DSDA with a mixture of3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and TMPDA;poly[2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropanedianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl]polyimide derived fromthe polycondensation reaction of 2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) with HAB; poly(DSDA-HAB-TMMDA)polyimide derived from the polycondensation reaction of DSDA with amixture of HAB and TMMDA; poly(BTDA-HAB-TMPDA) polyimide derived fromthe polycondensation reaction of BTDA with HAB and TMPDA;poly(BTDA-HAB-TMMDA) polyimide derived from the polycondensationreaction of BTDA with HAB and TMMDA;poly(BTDA-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-TMMDA)polyimide derived from the polycondensation reaction of BTDA with2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (APAF) and TMMDA.

The high hydrocarbon resistant chemically cross-linked aromaticpolyimide membrane described in the present invention can be fabricatedinto any convenient geometry such as flat sheet (or spiral wound), tube,or hollow fiber.

The present invention provides a method for the production of highhydrocarbon resistant chemically cross-linked aromatic polyimidemembrane by: 1) fabricating a blend aromatic polyimide polymer membranefrom a first aromatic polyimide containing carboxylic acid functionalgroups and a second aromatic polyimide containing hydroxyl functionalgroups; 2) chemical cross-linking of the blend aromatic polyimidepolymer membrane by heating the membrane at 100 to 300° C. under aninert atmosphere, such as argon, nitrogen, or vacuum. In some cases, amembrane coating step is added after step 1) and before step 2) bycoating the selective layer surface of the blend aromatic polyimidepolymer membrane with a thin layer of high permeability material such asa polysiloxane, a fluoro-polymer, a thermally curable silicone rubber,or a UV radiation curable epoxy silicone.

The invention provides a process for separating at least one gas from amixture of gases using the high hydrocarbon resistant chemicallycross-linked aromatic polyimide membrane described in the presentinvention, the process comprising: (a) providing a high hydrocarbonresistant chemically cross-linked aromatic polyimide membrane describedin the present invention which is permeable to said at least one gas;(b) contacting the mixture on one side of the high hydrocarbon resistantchemically cross-linked aromatic polyimide membrane described in thepresent invention to cause said at least one gas to permeate themembrane; and (c) removing from the opposite side of the membrane apermeate gas composition comprising a portion of said at least one gaswhich permeated said membrane.

The high hydrocarbon resistant chemically cross-linked aromaticpolyimide membrane described in the present invention is especiallyuseful in the purification, separation or adsorption of a particularspecies in the liquid or gas phase. In addition to separation of pairsof gases, the high hydrocarbon resistant chemically cross-linkedaromatic polyimide membrane described in the present invention may, forexample, be used for the desalination of water by reverse osmosis or forthe separation of proteins or other thermally unstable compounds, e.g.in the pharmaceutical and biotechnology industries. The high hydrocarbonresistant chemically cross-linked aromatic polyimide membrane describedin the present invention may also be used in fermenters and bioreactorsto transport gases into the reaction vessel and transfer cell culturemedium out of the vessel. Additionally, the high hydrocarbon resistantchemically cross-linked aromatic polyimide membrane described in thepresent invention may be used for the removal of microorganisms from airor water streams, water purification, ethanol production in a continuousfermentation/membrane pervaporation system, and in detection or removalof trace compounds or metal salts in air or water streams.

The high hydrocarbon resistant chemically cross-linked aromaticpolyimide membrane described in the present invention is especiallyuseful in gas separation processes in air purification, petrochemical,refinery, and natural gas industries. Examples of such separationsinclude separation of volatile organic compounds (such as toluene,xylene, and acetone) from an atmospheric gas, such as nitrogen or oxygenand nitrogen recovery from air. Further examples of such separations arefor the separation of He, CO₂ or H₂S from natural gas, H₂ from N₂, CH₄,and Ar in ammonia purge gas streams, H₂ recovery in refineries,olefin/paraffin separations such as propylene/propane separation, xyleneseparations, iso/normal paraffin separations, liquid natural gasseparations, C2+ hydrocarbon recovery. Any given pair or group of gasesthat differ in molecular size, for example nitrogen and oxygen, carbondioxide and methane, hydrogen and methane or carbon monoxide, helium andmethane, can be separated using the high hydrocarbon resistantchemically cross-linked aromatic polyimide membrane described in thepresent invention. More than two gases can be removed from a third gas.For example, some of the gas components which can be selectively removedfrom a raw natural gas using the high hydrocarbon resistant chemicallycross-linked aromatic polyimide membrane described herein include carbondioxide, oxygen, nitrogen, water vapor, hydrogen sulfide, helium, andother trace gases. Some of the gas components that can be selectivelyretained include hydrocarbon gases. When permeable components are acidcomponents selected from the group consisting of carbon dioxide,hydrogen sulfide, and mixtures thereof and are removed from ahydrocarbon mixture such as natural gas, one module, or at least two inparallel service, or a series of modules may be utilized to remove theacid components. For example, when one module is utilized, the pressureof the feed gas may vary from 275 kPa to about 2.6 MPa (25 to 4000 psi).The differential pressure across the membrane can be as low as about 70kPa or as high as 14.5 MPa (about 10 psi or as high as about 2100 psi)depending on many factors such as the particular membrane used, the flowrate of the inlet stream and the availability of a compressor tocompress the permeate stream if such compression is desired.Differential pressure greater than about 14.5 MPa (2100 psi) may rupturethe membrane. A differential pressure of at least 0.7 MPa (100 psi) ispreferred since lower differential pressures may require more modules,more time and compression of intermediate product streams. The operatingtemperature of the process may vary depending upon the temperature ofthe feed stream and upon ambient temperature conditions. Preferably, theeffective operating temperature of the membranes of the presentinvention will range from about −50° to about 150° C. More preferably,the effective operating temperature of the high hydrocarbon resistantchemically cross-linked aromatic polyimide membrane of the presentinvention will range from about −20° to about 100° C., and mostpreferably, the effective operating temperature of the membranes of thepresent invention will range from about 25° to about 100° C.

The high hydrocarbon resistant chemically cross-linked aromaticpolyimide membrane described in the present invention are alsoespecially useful in gas/vapor separation processes in chemical,petrochemical, pharmaceutical and allied industries for removing organicvapors from gas streams, e.g. in off-gas treatment for recovery ofvolatile organic compounds to meet clean air regulations, or withinprocess streams in production plants so that valuable compounds (e.g.,vinylchloride monomer, propylene) may be recovered. Further examples ofgas/vapor separation processes in which the high hydrocarbon resistantchemically cross-linked aromatic polyimide membrane described in thepresent invention may be used are hydrocarbon vapor separation fromhydrogen in oil and gas refineries, for hydrocarbon dew pointing ofnatural gas (i.e. to decrease the hydrocarbon dew point to below thelowest possible export pipeline temperature so that liquid hydrocarbonsdo not separate in the pipeline), for control of methane number in fuelgas for gas engines and gas turbines, and for gasoline recovery. Thehigh hydrocarbon resistant chemically cross-linked aromatic polyimidemembrane described in the present invention may incorporate a speciesthat adsorbs strongly to certain gases (e.g. cobalt porphyrins orphthalocyanines for O₂ or silver (I) for ethane) to facilitate theirtransport across the membrane.

The high hydrocarbon resistant chemically cross-linked aromaticpolyimide membrane described in the present invention also has immediateapplication to concentrate olefin in a paraffin/olefin stream for olefincracking application. For example, the high hydrocarbon resistantchemically cross-linked aromatic polyimide membrane described in thepresent invention can be used for propylene/propane separation toincrease the concentration of the effluent in a catalyticdehydrogenation reaction for the production of propylene from propaneand isobutylene from isobutane. Therefore, the number of stages of apropylene/propane splitter that is required to get polymer gradepropylene can be reduced. Another application for the high hydrocarbonresistant chemically cross-linked aromatic polyimide membrane describedin the present invention is for separating isoparaffin and normalparaffin in light paraffin isomerization and MaxEne™, a process forenhancing the concentration of normal paraffin (n-paraffin) in thenaphtha cracker feedstock, which can be then converted to ethylene.

The high hydrocarbon resistant chemically cross-linked aromaticpolyimide membrane described in the present invention can also beoperated at high temperature to provide the sufficient dew point marginfor natural gas upgrading (e.g, CO₂ removal from natural gas). The highhydrocarbon resistant chemically cross-linked aromatic polyimidemembrane described in the present invention can be used in either asingle stage membrane or as the first or/and second stage membrane in atwo stage membrane system for natural gas upgrading.

The high hydrocarbon resistant chemically cross-linked aromaticpolyimide membrane described in the present invention may also be usedin the separation of liquid mixtures by pervaporation, such as in theremoval of organic compounds (e.g., alcohols, phenols, chlorinatedhydrocarbons, pyridines, ketones) from water such as aqueous effluentsor process fluids. A membrane which is ethanol-selective would be usedto increase the ethanol concentration in relatively dilute ethanolsolutions (5-10% ethanol) obtained by fermentation processes. Anotherliquid phase separation example using the high hydrocarbon resistantchemically cross-linked aromatic polyimide membrane described in thepresent invention is the deep desulfurization of gasoline and dieselfuels by a pervaporation membrane process similar to the processdescribed in U.S. Pat. No. 7,048,846, incorporated by reference hereinin its entirety. The high hydrocarbon resistant chemically cross-linkedaromatic polyimide membrane described in the present invention that areselective to sulfur-containing molecules would be used to selectivelyremove sulfur-containing molecules from fluid catalytic cracking (FCC)and other naphtha hydrocarbon streams. Further liquid phase examplesinclude the separation of one organic component from another organiccomponent, e.g. to separate isomers of organic compounds. Mixtures oforganic compounds which may be separated using the high hydrocarbonresistant chemically cross-linked aromatic polyimide membrane describedin the present invention include: ethylacetate-ethanol,diethylether-ethanol, acetic acid-ethanol, benzene-ethanol,chloroform-ethanol, chloroform-methanol, acetone-isopropylether,allylalcohol-allylether, allylalcohol-cyclohexane, butanol-butylacetate,butanol-1-butylether, ethanol-ethylbutylether, propylacetate-propanol,isopropylether-isopropanol, methanol-ethanol-isopropanol, andethylacetate-ethanol-acetic acid.

EXAMPLES

The following examples are provided to illustrate one or more preferredembodiments of the invention, but are not limited embodiments thereof.Numerous variations can be made to the following examples that liewithin the scope of the invention.

Example 1 Preparation and Evaluation of Cross-Linked Aromatic PolyimideMembrane from Poly(6FDA-HAB) and Poly(DSDA-DBA-TMMDA) AromaticPolyimides

4.0 g of poly(6FDA-HAB) polyimide synthesized from polycondensationreaction of 6FDA and HAB monomers and 1.0 g of poly(DSDA-DBA-TMMDA)polyimide synthesized from polycondensation reaction of DSDA dianhydridewith DBA and TMMDA diamines with a DBA to TMMDA molar ratio of 1:1 weredissolved in 12.0 g of NMP and 10.0 g of 1,3-dioxolane solvents. Themixture was mechanically stirred for 2 hours to form a homogeneouscasting dope. The resulting homogeneous casting dope was allowed todegas overnight. The poly(6FDA-HAB)/poly(DSDA-DBA-TMMDA) blend membranewas prepared from the bubble free casting dope on a clean glass plateusing a doctor knife with a 20-mil gap. The membrane together with theglass plate was then put into a vacuum oven. The solvents were removedby slowly increasing the vacuum and the temperature of the vacuum oven.Finally, the membrane was heated at 200° C. under vacuum for 48 hours tocompletely remove the residual solvents. The driedpoly(6FDA-HAB)/poly(DSDA-DBA-TMMDA) blend membrane was heated at 300° C.under N₂ for 10 min to cross-link poly(6FDA-HAB) withpoly(DSDA-DBA-TMMDA) via esterification reaction between the carboxylicacid groups on poly(DSDA-DBA-TMMDA) and the hydroxyl groups onpoly(6FDA-HAB) to form the cross-linkedpoly(6FDA-HAB)/poly(DSDA-DBA-TMMDA) aromatic polyimide membrane.

The cross-linked poly(6FDA-HAB)/poly(DSDA-DBA-TMMDA) aromatic polyimidemembrane became insoluble in any organic solvents.

The cross-linked poly(6FDA-HAB)/poly(DSDA-DBA-TMMDA) aromatic polyimidemembrane is useful for a variety of gas separation applications such asCO₂/CH₄, H₂/CH₄, and He/CH₄ separations. The membrane was tested forCO₂/CH₄, H₂/CH₄, and He/CH₄ separations at 50° C. under 791 kPa (100psig) pure single feed gas pressure. The results show that thischemically cross-linked membrane has CO₂ permeance of 8.04 Barrers andhigh CO₂/CH₄ selectivity of 45.4 for CO₂/CH₄ separation. This chemicallycross-linked membrane has H₂ permeance of 33.5 Barrers and H₂/CH₄selectivity of 189 for H₂/CH₄ separation. This chemically cross-linkedmembrane also has He permeance of 41.8 Barrers and He/CH₄ selectivity of236 for He/CH₄ separation.

Example 2 Preparation of Cross-Linked Aromatic Polyimide Membrane fromPoly(DSDA-HAB-TMMDA) and Poly(DSDA-DBA-TMMDA) Aromatic Polyimides

2.5 g of poly(DSDA-HAB-TMMDA) polyimide synthesized frompolycondensation reaction of DSDA dianhydride with HAB and TMMDAdiamines with a HAB to TMMDA molar ratio of 1:2 and 2.5 g ofpoly(DSDA-DBA-TMMDA) polyimide synthesized from polycondensationreaction of DSDA dianhydride with DBA and TMMDA diamines with a DBA toTMMDA molar ratio of 1:3 were dissolved in 12.0 g of NMP and 10.0 g of1,3-dioxolane solvents. The mixture was mechanically stirred for 2 hoursto form a homogeneous casting dope. The resulting homogeneous castingdope was allowed to degas overnight. Thepoly(DSDA-HAB-TMMDA)/poly(DSDA-DBA-TMMDA) blend membrane was preparedfrom the bubble free casting dope on a clean glass plate using a doctorknife with a 20-mil gap. The membrane together with the glass plate wasthen put into a vacuum oven. The membrane was heated at 200° C. undervacuum for 48 hours to completely remove the residual solvents. Thedried poly(DSDA-HAB-TMMDA)/poly(DSDA-DBA-TMMDA) blend membrane washeated at 300° C. under N₂ for 10 min to cross-linkpoly(DSDA-HAB-TMMDA)/poly(DSDA-DBA-TMMDA) via esterification reactionbetween the carboxylic acid groups on poly(DSDA-DBA-TMMDA) and thehydroxyl groups on poly(DSDA-HAB-TMMDA) to form the cross-linkedpoly(DSDA-HAB-TMMDA)/poly(DSDA-DBA-TMMDA) aromatic polyimide membrane.

The invention claimed is:
 1. A chemically cross-linked aromatic polyimide polymer comprising a formula (I) wherein formula (I) is represented by the following formula:

wherein R is —COCH₃ or,

and mixtures thereof; X1, X2, X3, and X4 are selected from the group consisting of

and mixtures thereof respectively; Y2 is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; Y1 and Y3 are selected from the group consisting of

and mixtures thereof; n, m, n′ and m′ are independent integers from 2 to 500; n/m is in a range of 1:100 to 100:1; and n′/m′ is in a range of 1:100 to 100:1.
 2. The chemically cross-linked aromatic polyimide polymer of claim 1 wherein n/m is in a range of 1:10 to 5:1.
 3. The chemically cross-linked aromatic polyimide polymer of claim 1 wherein n′/m′ is in a range of 1:10 to 5:1.
 4. A chemically cross-linked aromatic polyimide membrane comprising the chemically cross-linked aromatic polyimide polymer of claim
 1. 5. The chemically cross-linked aromatic polyimide membrane of claim 1 comprising a coating selected from the group consisting of a polysiloxane, a fluoro-polymer, a thermally curable silicone rubber, and a UV radiation curable epoxy silicone. 